The T3 levels reported in the current study were slightly lower than previous reported levels from studies on kittiwakes in Kongsfjorden, Svalbard. Earlier studies have reported T3 levels in the range 3.10-4.16 ng/mL (Langset 2008, Ronning et al. 2008), while the mean levels in the present study were 2.7 and 2.4 ng/mL for 2008 and 2009 respectively. A possible explanation for the slightly lower T3 levels in the present study is the analytical procedures used in the different studies. Even though the T3 analyses in all these three studies were conducted using the same laboratory, the fact that they were analysed in different assays could result in inter-assay variability.
There was no significant difference in the T3 level between 2008 and 2009. Possible confounding factors, such as age and time of day of capture in the incubation period, were assumed to be similar in the two years. Seawater is abundant in iodine (Whitehead 1984), therefore, iodine deficiency in the diet and possible influences on the THs levels were not likely in the present study. Sex, BM and BC were controlled for in the statistical analyses,
and did not affect the T3 levels. In addition, the environmental conditions were approximately the same during the sampling period in respect of mean temperature for June 2008: 2.6 °C, 2009: 1.6 °C (Norwegian Metrological Institute 2008b, Norwegian Metrological Institute 2009b), and absence of sea-ice in the fjord (personal comments).
Langset (2008) reported that plasma T3 levels decreased with BM reduction, within one breeding season of kittiwakes breeding in Kongsfjorden. According to this, the T3 levels in 2009 should have been lower than in 2008, because of the lower BM of the kittiwakes in 2009 as compared to in 2008. The reason for not observing this relationship could be due to the extent of BM difference between 2008 and 2009. The mass loss of kittiwakes throughout the breeding season (~12%) is considerable greater compared to the BM difference observed between the two years of the present study. Furthermore, it is not expected to find the same relationships between different years as has been found within a season, due to physiological traits may change as they are dependent on environmental context and individual variations (Piersma and Lindstrom 1997).
In 2008 male kittiwakes had significantly higher T3 values compared to females. Previous studies have not reported sex-differences in the T3 levels, but sex-differences were detected in a recent study in other kittiwake colonies in Kongsfjorden (Welcker 2011, unpublished material). Sex-differences in hormone levels have been observed within many species in relation to reproductive and parental care (Lormee et al. 2000, Lormee et al. 2003). However, T3 is not directly linked to the parental behaviour during the breeding season, such as luteinizing hormone, steroid hormones, prolactin and corticosterone (Angelier et al. 2007, Angelier et al. 2009, Goutte et al. 2010). The higher T3 level in males may be a result of a higher BMR in males compared to female kittiwakes. However, the research done on BMR in kittiwakes is so far limited to females (Bech et al. 1999, Ronning et al. 2008). Thus, sex-differences in BMR of kittiwakes have never been studied (Bech 2011, personal comments).
In the present study there was a positive relationship between wing length and T3 levels (Figure 3.4.4). Male kittiwakes have naturally longer wings than females (Appendix D; Table D.1 and D.2), and in the present study the males had higher T3 levels. Thus, the correlation between T3 and wing was most likely caused by the sex-differences in the T3 levels (Figure 3.4.3.b) of the kittiwakes. Thus, wing length, as a measure of body size, appeared to be a determinant biometric variable for plasma T3 concentrations, and not BC or BM.
There were no associations between the whole blood concentrations of POPs and the plasma T3 levels in the kittiwakes (Appendix G; Table G.4). Several studies have linked increasing concentrations of POPs to decreasing levels of THs (Braathen et al. 2004, Skaare et al. 2001, Sormo et al. 2005, Verreault et al. 2004). However, the associations between POPs and THs reported in previous studies have primarily been between POPs and T4 (Dawson 2000, Verreault et al. 2004). In the few studies reporting associations between POPs and T3, the reported effects include both positive and inverse associations between POPs and T3 (Braathen et al. 2004, Sormo et al. 2005, Verreault et al. 2004). For instance, Verreault et al.
(2004) reported elevated plasma T3 levels in glaucous gulls exposed to high levels of POPs.
On the contrary, the relationship between PCBs and T3 was negative in polar bears at Svalbard, and the authors suggested T3 to be more susceptible for PCB-induced alterations than T4 (Braathen et al. 2004). In addition, several of the studies reporting associations between TH and POPs are laboratory based studies on rodents or field studies on mammals (Dawson 2000). Unlike mammals, where TTR is the major transport protein, T4 is mainly associated with the transport protein albumin in avian species (McNabb 2000). Thus, competitive binding between POPs and T4 for binding sites on albumin has not necessarily the same extent in birds, as competitive binding between POPs and THs for TTR in mammals.
The less susceptibility for POP-interference with THs transport protein in birds could be the reason for the unaffected T3 levels in the present study. It is, however, possible that if also T4 was analysed in the present study, an effect on the TH homeostasis of the kittiwakes could have been found.
Verreault et al. (2004) reported oxychlordane and HCB to be the most prominent compounds in terms of their effect on the THs levels in glaucous gulls at Bear Island. In contrast to this, there were no associations between these contaminants and T3 in the present study. This was despite the higher concentrations of HCB and oxychlordane in 2009 as compared to in 2008.
It is possible that the lack of these hypothesized inverse relationships is due to the relative low concentrations of POPs in kittiwakes compared to in glaucous gulls (Borga et al. 2001, Hop et al. 2002). Several studies have failed to prove strong associations between POPs and THs in species with lower positions in the arctic food web (Nordstad 2009, Nøst 2009). Since there have been reported several effects of POPs on THs in glaucous gulls (Verreault et al. 2004) and herring gulls (McNabb and Fox 2003), it can be assumed that the levels of POPs in seabirds occupying lower trophic levels may not be sufficient enough to exceed critical threshold levels for POPs-induced alterations on the THs homeostasis. In addition,
Ucán-Marin et al. (2010) reported brominated compounds to be more forceful competitive ligands to gull recombinant albumin and TTR relative to both T3 and T4, compared to chlorinated compounds. Considering the low levels of chlorinated compounds in kittiwakes observed in the present study, and the fact that avian species have been reported to be less susceptible to POP-induced changes to TH levels compared to mammals, the present study suggests that the current environmental concentration exposures of POPs in the Arctic are not sufficient to affect the T3 levels of kittiwakes.